Tateki Fujiwara

Effect of the Earth's rotation on the circulation in regions of freshwater influence

Recent surveys in various regions of freshwater influence have shown considerably different flow patterns in each region. An analytical model including viscous effects and the Earths rotation is proposed to examine the along-channel flow pattern and to explain the differences. Model results show that the flow pattern is strongly dependent on the Ekman number E. With a large Ekman number (E > 1) the system is governed by gravitational circulation, and thus horizontal density gradients in along-channel direction are important. The whole water column is in the Ekman layer and consequent jet inflow penetrates the surface over a deep depression if the bottom topography varies in a cross-channel direction. With an intermediate Ekman number (E ∼ 0.1) this jet inflow concentrates in the lower layer in which the viscosity still plays an important role. The flow in the upper layer is, on the other hand, determined by the geostrophic balance. The contribution by the geostrophic flow becomes larger so that the cross-channel density gradients are important when die Ekman number is small (E < 0.01). Since the Ekman layer is clung to the bottom, the jet inflow exists only in the thin bottom layer. The hydrographic and acoustic Doppler current profiler surveys were conducted in Ise Bay, Japan. Both the density structure and the flow pattern were different from those observed in many drowned river valleys. A strong jet inflow existed in the lower layer over the depression while the flow in the upper layer suggested anticyclonic circulation. The estimated Ekman number is 0.07 in the bay and thus the observed pattern is consistent with the model result when E = 0.1.

Anti-cyclonic circulation driven by the estuarine circulation in a gulf type ROFI

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. Baroclinic residual circulation processes are examined in gulf type Regions Of Freshwater Influence (ROFIs), which have large rivers discharging into a rounded head wider than the Rossby internal deformation radius. Theoretical and observational investigations concentrate on Ise Bay, Japan, with supporting data from Osaka Bay and Tokyo Bay. Simplified analytical solutions are derived to describe the primary features of the circulation. Three dimensional residual current data collected using moored current meters and shipboard acoustic doppler current profilers (ADCPs), satellite imagery and density structure data observed using STDs, are presented for comparison to the theoretical predictions. There are three key points to understanding the resulting circulation in gulf type ROFIs. First, there are likely to be three distinct water masses: the river plume, a brackish upper layer, and a higher salinity lower layer. Second, baroclinic processes in gulf type ROFIs are influenced by the Earths rotation at first order. Residual currents are quasi-geostrophic and potential vorticity is approximately conserved. Third, the combined effects of a classical longitudinal estuarine circulation and the Earths rotation are both necessary to produce the resulting circulation. Anti-cyclonic vorticity is generated in the upper layer by the horizontal divergence associated with upward entrainment, which is part of the estuarine circulation. The interaction between anti-cyclonic vorticity and horizontal divergence results in two regions of qualitatively different circulation, with gyre-like circulation near the bay head and uniformly seaward anti-cyclonicly sheared flow further towards the mouth. The stagnation point separating the two regions is closer to (further away from) the bay head for stronger (weaker) horizontal divergence, respectively. The vorticity and spin-up time of this circulation are −(f−ω l )/2 and h/2w 0 , respectively, where f is the Coriolis parameter, ω l is the vorticity of the lower layer, h is the depth of the upper layer and w 0 is the upward entrainment velocity across the pycnocline. Under high discharge conditions the axis of the river plume proceeds in a right bounded direction, describing an inertial circle clearly seen in satellite images. Under low discharge conditions the river plume is deflected in a left bounded direction by the anti-cyclonic circulation of the upper layer.

Tidal-jet and vortex-pair driving of the residual circulation in a tidal estuary

Abstract Many tidal estuaries are composed of narrow constrictions and wide basins, forming a “strait-basin system”. A typical example of such a system is that of the Akashi Strait and Osaka Bay in the Seto Inland Sea, Japan. In this system, tidal currents in the straits are much stronger than those within the basins. Therefore the mass transport processes are advection rather than diffusion and governed by strongly non-linear effects. In order to reveal the flow and transport mechanism in the strongly non-linear effect, field observations have been conducted. The observations include flow-visualization experiments by the simultaneous use of several shipboard acoustic Doppler current profilers. In the strait-basin system, the tidal-jet from the strait generates a moving vortex-pair, which is smaller than the tidal excursion. This vortex-pair carries vorticity originally generated at the separation point in the strait far into the basin during each tidal cycle. The moving vortex replenishes the vorticity of the basin-wide residual circulation, which is larger than the tidal excursion. This residual circulation further acts on the trajectories of the moving vortices. Generation of moving vortices from a tidal-jet is not unique but common and essential phenomena in many strait-basin systems. These systems are filled with vortices.

Circulation and cold dome in a gulf-type ROFI

Abstract Recent surveys in Ise Bay, which is a major gulf-type region of freshwater influence in Japan, reveal that a cold dome is often observed in the stratified season. To elucidate the formation mechanism of the cold dome, detailed hydrographic and ADCP surveys were conducted in August 1995. The results show that a cold (T

Formation of sand banks due to tidal vortices around straits

Sand banks around straits are used as a commercial fishing ground. In order to clarify the mechanism of sand bank formation, the Lagrangian method was used to measure currents and turbidity around the banks in the Neko Seto Sea in the Seto Inland Sea of Japan. A neutrally buoyant float released in the Neko Seto Strait at the maximum tidal flow stage was engulfed in a pair of tidal vortices and moved around one of the sand banks. The vertical distribution of turbidity, which was measured by the vessel moving with the neutral float, showed an extremely high turbidity in the bottom layer of this bank area. According to the analysis of these observational data, the process of sand bank formation around straits is as follows. The tidal vortex transports water mass with suspended materials (including sand) which are whirled up at the bottom by the tidal jet. In the decaying stage of the vortex, the materials in the bottom layer are gathered in the central part of the vortex by the secondary convergent flow in the vortex. Among these materials, a large-size sand particle with a high critical erosion velocity accumulates at the bottom and forms banks. The distribution of bottom sediment and the thickness of alluvium support this result.

Sea surface temperature distribution and its variability across the Tsushima Strait

The sea surface temperature distribution across the Tsushima Strait was monitored over a one-year period on board the ferry Kampu which runs between Shimonoseki, Japan and Pusan, Korea. A cold water region is always observed just near the Korean Coast, and a sharp temperature front is always present in the western channel. A temperature maximum or a warm core is usually found just on the southeast side of the front. The position of the warm core exhibits large short period fluctuations, but no significant seasonal variation is found. Sudden temperature increases followed by sudden temperature decreases are frequently observed in the temporal variation curves at fixed positions during the warming season from April to August. Such events are related to temperature maxima found sporadically in the temperature distribution in the eastern channel during this season, and seem to be caused by warm water intrusion into the Tsushima Strait from the East China Sea.

Flow characteristics governing the distribution of water and bottom quality in a semi-enclosed sea

Abstract Semi-enclosed seas are generally composed of narrow straits and wide basins, forming a strait-basin system. Tidal currents through the strait have a jet-like characteristic. This tidal-jet flows into a stagnant basin and forms a pair of tidal-vortices (vortex dipole). In the strait-basin system, sea area is divided into two regions: well-mixed region around the strait (WM-region), and stagnant and stratified region in the inner part of the basin (SS-region). The WM-region is the region where highly turbulent water comes from the strait, and the SS-region is the opposite of it. Whether a point is included in the WM-region or the SS-region does not depend on a local parameter, but on the hysteresis of the water. In the WM-region, mass and volume transport are driven by the tidal-vortices. The strait-basin system is characterized by “the high energy tidal-jet flowing from the strait into the stagnant basin with a certain period and intensity”. The concept of this system is new and could be interesting in future studies of mass transport in semi-enclosed seas.

Nutrient Dynamics in the North Basin of Lake Biwa: I. Changes in the Vertical Distribution of Nutrients Due To an Internal Surge Induced by a Strong Typhoon

Abstract The north basin of Lake Biwa is thermally stratified from April to November. During this period in 1993, the concentration of nitrate in the epilimnion, which was 10 to 100 times greater than that of ammonia or nitrite, decreased with time due to uptake by phytoplankton while in the hypolimnion the concentration of nitrate was increased. In comparison, the concentration of total phosphorus (TP) was greater in the epilimnion than in the hypolimnion, and correlated with concentration of chlorophyll a. In September in 1993, a strong typhoon (9313) induced large internal surges in the lake causing drastic changes in the vertical distribution of nutrients in the north basin. The vertical mixing caused increases in nitrate concentration at depths of 1 m and 10 m which corresponded to the re-supply that usually takes 15 days and 20 days, respectively, to be removed from the epilimnion. These changes persisted well after the typhoon had abated.

Structure of the strong tidal jet in the naruto strait

In order to clarify the structure of the strong tidal current at the Naruto Strait in the Seto Inland Sea of Japan, the sea-level values were observed in the strait and the current measurements were made with an Acoustic Doppler Current Profiler (ADCP).The tidal volume transports for M2 and S2 tides were about 74×103 and 26×103 m3 sec−1, respectively. The horizontal profile of the velocity at the phase of the strong tidal current compares favorably with a theoretical profile of the two-dimensional steady turbulent jet except for the side parts of the profile. Moreover, the entrainment rate of the surrounding water into the strong tidal jet was estimated from the difference of mass flux between two cross-sections at the strait, the entrainment rate and entrainment constant for both the northward and southward flows being about 1.3–2.5×10−4m−1 and about 0.03–0.05, respectively.

Energy balance of the tidal current in the Obatake Seto strait

Current measurements made with a Doppler Current Profiler were taken along cross sections in the Obatake Seto Strait, a small strait in the western part of the Seto Inland Sea.The energy loss between two cross sections of the strait was estimated using the control volume analysis of the stationary one-dimensional energy equation. The large energy loss observed in the strait cannot be explained in terms of general bottom friction in the coastal region. The loss is attributed to the form loss, which is accompanied by internal turbulence and vortices generated by the specific topography of the strait, in addition to the loss by friction. This is supported by the fact that the energy loss in the strait differs, depending upon the current direction.